1. Field of the Invention
The present invention relates to a method and apparatus for controlling a drafting unit for use in the textile industry.
2. Description of Background and Other Information
Various known devices and control systems for equalizing textile slivers are based on the draft thereof. Usually the slivers are doubled and equalized in a two-stage drafting process, i.e., a preliminary draft and a main draft. The object of the drafting process is to produce a substantially uniform sliver. The drafting unit is supplied with irregular or non-uniform slivers, which are to be combined at the outlet into a sliver having a substantially uniform predetermined cross-section. It is necessary, therefore, to control the drafting process. There are various prior-art drive means and automatic control devices for addressing this problem.
For example, open and closed control loops and combinations thereof are known for measuring the sliver cross-section on the outlet side or delivering measurable variables corresponding to the cross-sections at the inlet, so as to control the draft by the unit, via final control elements. The problems of using open control loops are known in automatic control engineering in general and in the control of drafting processes in particular. The difficulties result from the transit time between the measuring point and the final control element and also through lack of feedback information. Further, in drafting units there are additional problems specifically related to the drafting process.
On the other hand, closed control loops also present difficulties, e.g., because short-term disturbances cannot be compensated due to the dead time between the measurable variable and the final control element. There are also difficulties in measurement technology with regard to the processing of the sliver.
European Patent Application No. 0 176 661, for example, discloses a method and apparatus for optimizing the drafting process in controlled drafting units in the textile industry, including an open and a closed control loop. The specification, starting from the aforementioned difficulties, proposes providing a feedback path between the output-side measuring point and the control circuit, and using the feedback information to influence the control parameters. The control concept, which is basically known, comprises computerized coordination of the input and output measurable variables by means of cross-correlation.
In the prior art, account is taken of both an output-end and an input-end measurable variable, which can in principle result in improved control of the uniformity of a sliver. On the other hand, the known devices and methods do not take account of the special features of control engineering and particularly of measurement technology, so that these control systems, in spite of additional steps, give only limited results.
Another result of these special features in the control of drafting units is that the installations need to be monitored during operation and some parameters need to be manually adjusted or corrected. One particular disadvantage is that dead time and/or amplification usually are required to be set manually and adjusted during operations. This requires continuous monitoring by an operator.
Additional problems are caused by the prior art design of the drive means and devices. Disturbances and fluctuations caused by the drives are taken into account by the known control systems, but it has been found that if the corresponding disturbances are to be taken into account in a single main or overall control system, it is necessary to compensate very large control ranges, and this overloads conventional equipment.
Another disadvantage is that the entire computer load is concentrated and the timing of the control system is not optimized.